When specular reflection is desired, mirrors and other reflecting surfaces have been traditionally manufactured by coating a well-polished surface of glass or metal with a reflecting metal like gold, copper, silver or aluminum. In order to avoid deterioration with time, these highly polished surfaces are usually covered with protective coatings. In some unique applications, particularly when the absorption of the coating itself tends to degrade the reflection efficiency of the mirror, an extremely durable reflective layer of rhodium has been used as well. For reflection in some parts cf the electromagnetic spectrum, highly reflective insulators like the titanates, zincates and zirconates have also been used. When non-specular, or diffuse, reflection was desired, the substrate was often purposely etched and then coated with the reflecting substance, chosen from the above mentioned reflectors. The etching provides the substrate with its dull appearance, and as a result non-specular, or Lambertian reflection, occurs (a Lambertian optical source is characterized by the fact that the intensity of its illumination is equal in all directions).
One of the shortcomings of traditional mirrors and reflectors used in high energy density applications is that even the best of said mirrors, and/or reflectors, absorbs at least one percent of the incident light. This imposes limitations and restrictions on the application of traditional mirror technology, especially where multiple reflections are desired or when the reflected beams have extremely high energy density, as in laser technology.
The known superconductors have never been considered for mirror and other reflective applications, despite the fact that they have conductivities which are better than normal conductors such as gold, silver and copper. In classical superconductors, the high conductivity results from pairing of the charge carriers. This pairing involves a binding energy that, for most classical superconductors, is less than 3 milli-electronvolts. Thus, when electromagnetic radiation with wavelengths shorter than about 0.4 mm impinges on such classical superconductors, it is absorbed and decouples the paired charge carriers. For most optical applications, including the infrared wavelengths, superconductors with much higher pairing energies are needed in order to be able to reflect at much shorter wavelengths.
Until recently, it was believed that superconductivity above 23 K. (and band gaps in excess of 3 milli-electronvolts) was not possible. This belief was rooted in the theoretical work now named the BCS theory (Bardeen, Cooper and Schrieffer) which predicted such an upper limit. As a result, no research in the field of superconducting mirrors and reflectors has been known heretofore.
The temperature at which superconductivity occurs in a superconductor (in the absence of any external magnetic fields) is termed the critical temperature of that superconductor. In the early 1970's a number cf theoretical proposals were presented, suggesting that the critical temperature for superconductivity could be increased (V. L. Ginzburg, Usp. Fiz. Nauk. 101, 185 (1970)), (D. Allender, J. Bray, J. Bardeen, Phys. Rev. B8, 4433 (1973)), but the lack of any discoveries of superconductivity above 23 K., solidified the belief that indeed this critical temperature could not be exceeded. A significant experimental breakthrough in high temperature superconductivity (critical temperatures in excess of 23 K.) was provided in November 1986 by Bednorz and Muller when they published a tentative disclosure of high temperature superconductivity (Georg Bednorz and Alex Muller, Z. Phys. B64, 189 (1986)). Rapid confirmation by others was soon obtained. For instance, a report cites a critical temperature above 30 K. for La.sub.2-x Ba.sub.x CuO.sub.4-y, (H. Takagi, S. Uchida, K. Kitazawa, S. Tanaka, Jpn. J. Appl. Phys. 26, L123 (1987)) Confirmation of a critical temperature of 93 K. was reported by Chu for a yttrium-barium-copper oxide ceramic (M. K. WU, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, Phys. Rev. Lett. 58, Mar. 2, 1987, p. 908.) This material has since been called the 123 compound, and has served as a model for advanced research in the field.
During 1987 and 1988, a number of families of high temperature superconductors have been discovered, with confirmed critical temperatures all the way to 162 K. These materials are usually ceramics containing copper (whose apparent valence state appears to be trivalent), an alkaline earth metal (Ca, Sr, or Ba) and a rare earth including Yttrium. Most of these superconductors have shown some degree of anisotropy in their properties. Therefore, it was significant when a superconducting ceramic of cubic symmetry, having a critical temperature above 23 K. (specifically 30 K.) was discovered based on a complex oxide of Ba, K and Bi. This superconductor was the first high temperature superconductor which did not contain copper in its composition. The implication is that the occurrence of high temperature superconductivity may be more widespread than has been realized to date. In addition, amorphous high temperature superconductors have also been reported, based on the bismuth compounds in which some of the bismuth was replaced with lead. The critical temperatures and critical current of these amorphous superconductors are somewhat lower than those of their crystalline counterpart.
There are some scattered reports cf superconductivity above 162 K. For instance, R. G. Kulkarui has reported that oxides having an approximate composition 0.5CaO.multidot.0.5ZnO.multidot.Fe.sub.2 O4. become superconducting at about this temperature. Ogushi has also reported room temperature superconductivity in yet ill-defined niobium strontium lanthanum oxides. While these reports have yet to be confirmed independently by other researchers, it is reasonable to expect that superconductors with critical temperatures near to room temperature will soon be obtained, having electron pairing energies much higher than known heretofore.
In classical superconductors, the optical band gap has been found to be equal to the electronic band gap (as measured on a Josephson junction), and has values of about 3.5 kT.sub.c. However, in some of the high temperature superconductors, I have determined that the observed critical temperature and the "optical" critical temperature are not equal, and that the said optical critical temperature, which I have termed the "virtual critical temperature", can be much larger than the thermodynamic critical temperature. As a result, the optical band gap can be much higher than 3.5 kT.sub.c. I further find that fore wavelengths longer than their optical band gap, these superconductors can reflect electromagnetic radiation more efficiently than normal metals, and with smaller losses of reflected energy.
I have also determined that when the superconductors are quenched by chosen means into their normal state, they combine absorption, reflection and transmission of light at ratios that depends on the physical properties of their respective normal states. Thus, superconducting materials will be found that have charge carriers with virtual critical temperatures (pairing energies) in excess of 2 electron-volts. This makes possible mirrors capable of reflecting electromagnetic radiation in the infrared as well as in the visible part of the spectrum. This postulation is based not only on the classical scaling of the charge carriers' binding energy with temperatures of 3.5 kT.sub.c which in themselves would be insufficient to reach the infrared range (unless critical temperatures in excess of 500 K. are achieved), but is based particularly on my discovery that the virtual critical temperature can be more than twice the thermodynamic critical temperature.
A further tenet of the instant invention is that some of these high temperature superconductors have the unique property of being insulators in their normal state, and basically transmit electromagnetic radiation (at least within a specific band that depends on the electronic state of the normal state in a manner well known in the art). To differentiate between the different classes of superconductors and clearly define classes that are suitable for the practice of this invention, I have classified superconductors according to the nature of their corresponding normal state. Classical BCS superconductors are usually metallic in their normal state. Therefore, they belong to a class that I have denoted as (SC,M), namely, that below and above their critical temperature a superconducting and a metal phase exists, respectively. A few examples of this class of superconductors are mercury, niobium and its A15 intermetallic compounds with tin and germanium.
The newer class of ceramic superconducting oxides, that are semimetals or semiconductor in their normal state, thus belong to a class that I have denoted as (SC,S) in a similar manner. Examples of superconductors belonging to this class are the 123 compounds and the bismuth-based oxide superconductors.
Finally, the last class of superconductors are insulators in their normal state and thus belong to a class that we denote, (SC,I). An example of this group is Kulkarui's superconducting spinel-like compound, having an approximate composition of 0.5CaO.multidot.0.5ZnO.multidot.Fe.sub.2 O.sub.4.
I have determined that some superconductors belonging to the last two classes have virtual critical temperatures that are higher than the actual, or thermodynamic, critical temperature. I have further established that the differential is expected to be much larger for the superconducting oxides of the class (SC,I) with a normal insulating state. Thus, the present invention concerns only mirrors made with or of superconductors belonging to the (SC,S) class, wherein their normal state is that of a semiconductor, and/or the (SC,I) class, namely wherein their normal state is an insulator.